There are certain known relay valves, one of which is shown and disclosed in published German Patent Application DE-OS 27 03 940. A relay valve of this type is normally located in pneumatic braking installations for vehicles between reservoir and pneumatic consumers, namely, the brake cylinders. When pressurized with a control pressure which is supplied, for example, by a pedal-operated brake valve, an outlet valve connecting a working connection of the valve with the atmosphere is closed, and an inlet valve connecting a reservoir with the working connection is opened. Thus, there is a rapid buildup of pressure to the consumers. When the control pressure decreases or disappears, the inlet valve is closed and the outlet valve is opened, whereupon the consumers are rapidly partly or entirely exhausted to atmosphere.
On such a relay valve, the working pressure exerted and the control pressure are in a specified pressure ratio to one another based on the design of predetermined values established by the ratio of surface areas on the relay piston.
There are applications in which the consumers can function with the design pressure ratio only in a limited control pressure range, and beyond that require an increase in the working pressure, up to the full available supply reservoir pressure, which is graduated and more rapid than that specified by the design pressure ratio.
An example of consumers of this type for motor vehicles with load-controlled brake force controllers is represented by requirements in several countries regarding such braking action. The requirement in these countries is that, in emergency situations, the reservoir pressure available in the vehicle must be able to be released to the brake cylinders in controllable stages. Ordinarily, the load-controlled brake force controllers have a control action with a predetermined correspondence between load status or dynamic axle load and the working pressure (i.e. brake pressure) released, and therefore cannot be used for applications of the type described above.